1. Field of the Invention
This invention generally relates to the measurement of power consumption and more specifically for calibration and error correction of split core current transformers used in non-intrusive and self-powered measurement devices.
2. Prior Art
In a typical electricity distribution system, power is provided through a main circuit breaker and a device for measurement of the power consumption of the entire electrical network connected thereto. However, typically, the main power line is then connected to a plurality of circuit breakers, each feeding a smaller section of the electrical network with its specific power requirements. The circuit breaker is adjusted to the amount of maximum current that may be used by this electrical sub-network. In industrial and commercial applications, hundreds of such circuit breakers may be installed, each controlling a section of the electrical network. Even in smaller locations, such as a house, it is not unusual to find tens of circuit breakers controlling various electrical sub-networks.
Non-intrusive measurement of current through a power line conductor has well known principles. A current transformer (CT) of sorts is created that comprises the primary winding as the power line conductor and the secondary providing an output current inversely proportionate to the number of windings. Typically such systems are used for measuring currents in very high voltage or current environments, for example, as shown in Gunn et al. in U.S. Pat. No. 7,557,563. These types of apertures are useful for main power supplies. Using such devices, or power meters for that matter, is deficient for the purposes of measuring relatively low currents in an environment of a plurality of circuit breakers. Providing wireless telemetry on a singular basis, such as suggested by Gunn et al., and other prior art solutions, suffers from deficiencies when operating in a noisy environment.
Split cores are used when non-intrusive installation is required, i.e., there is a need or requirement not to disconnect electrical wires when installing a CT. A split core is comprised of two parts that are positioned to fit each other to create a full core. A major problem with split core CT's is that the measurement result is highly sensitive to the air-gap that always exists between the two parts of the core. Due to production tolerances, there may be significant variations in the measurement result between seemingly like cores due to miniature variations (at an order of magnitude of tens of micro-meters) between their air-gaps. As a result, such CT's cannot be practically used in applications requiring a high level of accuracy, such as, for example, revenue grade utility metering. Other sources of inaccuracy in CTs (not necessarily limited to those having a split core) are the number of winding which varies due to tolerances of the winding process, variations in the core cross section dimensions and magnetic material properties.
When using CT's to measure current, such inaccuracies are manifested in variations in the coupling factor, i.e., the ratio between primary and secondary signal amplitude, phase error, i.e., the phase shift between primary and secondary signal amplitudes, and temperature error coefficients, i.e., measurement errors due to variations in temperature. Several solutions have been proposed to the problem some of which involve manually configurable mechanical parts (such as variable resistors) assembled as part of the CT to tune the device. Others proposed solutions that include a matrix of passive components such as resistors, capacitors, and/or inductors, assembled as part of the CT. Such passive components are added and/or removed to tune the device.
There is a need in the art that is now developing, resulting from the move toward energy conservation and need to increase operational efficiency to enable measurement and analysis of power consumption on a fine granularity at the device or circuit level. It would be advantageous if a solution may be provided for calibration of CT's that is simple, cost effective and easy to maintain over long periods of time where the equipment is installed.